The invention pertains to circuit board test, and more specifically, to the test of circuit boards on which one or more opto-electronic devices are installed.
Today""s high-speed communication systems often comprise a mix of electrical and optical subsystems 100, 102 (FIG. 1). While electrical subsystems propagate signals via electrons, optical subsystems propagate signals via photons (i.e., lightxe2x80x94denoted in the figures by the Greek lambda xe2x80x9cxcexxe2x80x9d symbol). An interface comprising one or more opto-electronic devices 104 is therefore needed to convert electrical signals to optical signals and/or vice versa.
An opto-electronic (OE) device may assume one or both of two roles: that of transmitter 106 (i.e., a device which converts electrical signals to optical signals), or that of receiver 108 (i.e., a device which converts optical signals to electrical signals). In the case of an OE transmitter, electrical signals propagated over wires, cables and/or printed circuit board traces enter the electronic portion of the OE transmitter. The electrical signals are then converted to photonic signals that are output from the optical portion of the OE transmitter. The photonic signals propagate over fiber optic cables and/or through air. In the case of an OE receiver, photonic signals propagated over fiber optic cables and/or through air enter the optical portion the OE receiver. The photonic signals are then converted to electrical signals that are output from the electrical portion of the OE receiver.
With respect to OE transmitters, the electronic portion of a transmitter will often implement monitor and controller functions that 1) maintain the quality of signals output from the optical portion of the transmitter, and 2) control safety factors relating to operation of the optical portion of the transmitter.
A common safety factor which is considered during operation of an OE transmitter is eye safety. The wavelengths of light emitted by an OE transmitter can be dangerous to the human eye, especially when viewed over extended periods (that is, extended periods with respect to the operation of the transmitterxe2x80x94such as a few milliseconds). Eye safety constraints can be respected, however, by using the electronic portion of an OE transmitter to monitor and control the steady-state power output of the transmitter""s optical portion. Typically, when steady-state power cannot be controlled (e.g., due to a transmitter defect), or when total power output exceeds a defined threshold, then the electronic and/or optical portions of the transmitter automatically shut down and xe2x80x9clock outxe2x80x9d the optical portion of the transmitter. Upon xe2x80x9clock outxe2x80x9d, the optical portion of the transmitter will only xe2x80x9cunlockxe2x80x9d (i.e., restart) upon receipt of a reset sequence from the electronic portion of the transmitter.
When OE devices are soldered to a printed circuit (PC) board, they present testing problems. For instance, consider an OE transmitter having an electronic portion soldered to a PC board, but having an optical portion that serves as a board output. In order to test such a transmitter, a tester, such as an Agilent 3070 Board Tester (manufactured by Agilent Technologies, Inc. of Palo Alto, Calif., USA), must not only be capable of stimulating the electrical inputs of the transmitter, but it must also be capable of capturing a photonic response from the transmitter. This is not a standard capability of board test equipment. In fact, even if a custom photonic receptor can be coupled to (or placed in the line of transmission of) a transmitter""s photonic output, it can be difficult to devise a test that even generates a response to be captured. This is particularly so with respect to xe2x80x9cin-circuitxe2x80x9d, xe2x80x9cstructuralxe2x80x9d, or xe2x80x9cscanxe2x80x9d testing, wherein the design of test patterns which obey the stimulation rules of a transmitter""s electrical inputs may be difficult. If stimulation rules are not obeyed, the afore-mentioned monitoring and control functions of a transmitter""s electronic portion may cease to operate (resulting in potential eye hazards), or more likely, the monitoring and control functions will operate all too well, and the optical portion of a transmitter will be disabled. In the latter case, further tests of the transmitter are not possible until the transmitter is reset. Furthermore, if a good transmitter is inadvertently disabled, a tester may fail to realize this and merely identify the transmitter as failed without attempting a reset.
Resetting an OE transmitter is a problem unto itself. First, depending on which external inputs and/or internal nodes a tester is designed to stimulate, the launch of a reset sequence may not be possible. Second, assuming that the launch of a reset sequence is possible, the test pattern(s) which are needed to launch such a sequence may not be known, or may only be derived after significant effort. Finally, even in a best case scenario, where the launch of a reset sequence is possible, and the test pattern(s) for launching the reset sequence are known, the launch may consume a great number of test cycles.
Next, consider an OE receiver having an electrical portion soldered to a PC board, but having an optical portion that serves as a board input. In order to test such a receiver, a tester must not only be capable of capturing an electrical response from the receiver, but it must also be capable of providing a photonic input to the receiver. Again, this is not a standard capability of board test equipment. In fact, even if a custom photonic transmitter can be coupled to (or placed in front of) a receiver""s photonic input, a tester may find it difficult to generate the type of photonic inputs that the OE receiver expects. This is especially so given that many testers are unable to provide test patterns xe2x80x9cat speedxe2x80x9d for an electronic device, and OE devices typically operate at even higher speeds.
Finally, consider the case of an OE pair (i.e., an OE transmitter that is optically coupled to an OE receiver). In such a case, a tester no longer needs to transmit or receive photons. However, a tester is still charged with understanding, producing and monitoring the electronic protocols at either end of the pair.
The inventors have devised new methods and apparatus pertaining to boundary-scan testing of opto-electronic devices.
On the transmit side, an exemplary system may comprise an opto-electronic transmitter and a boundary-scan cell. The opto-electronic transmitter comprises an electrical input and an optical output. The boundary-scan cell comprises a signal generator. The signal generator, in turn, comprises an input, an output, and some logic. The signal generator input is coupled to receive test data that is shifted into the boundary-scan cell, and the signal generator output is coupled to provide conditioned test data to the electrical input of the opto-electronic transmitter. The logic is coupled between the signal generator""s input and output and serves to generate the afore-mentioned xe2x80x9cconditioned test dataxe2x80x9d in conformance with 1) the shifted test data, and 2) at least one constraint for operating the opto-electronic transmitter.
On the receiving side, an exemplary system may comprise an opto-electronic receiver and a boundary-scan cell. The opto-electronic receiver comprises an optical input and an electrical output. The boundary-scan cell comprises a signal detector. The signal detector, in turn, comprises an input, an output, and some logic. The signal detector input is coupled to receive a response from the electrical output of the opto-electronic receiver, and the signal detector output is coupled to provide an evaluated response to the shift register element. The logic is coupled between the signal detector""s input and output and serves to generate the afore-mentioned xe2x80x9cevaluated responsexe2x80x9d after monitoring the signal detector input over a period of time.
Finally, a method for testing opto-electronic devices may comprise 1) shifting test data into a first boundary-scan cell, 2) launching a test from the first boundary-scan cell by outputting the shifted test data to a signal generator, and 3) capturing a response to the test. The signal generator provides conditioned test data to an opto-electronic transmitter, in response to the shifted test data and at least one constraint for operating the opto-electronic transmitter.